An electrochemical capacitor has improved output properties and life properties in comparison with a battery. Based on such properties, storage power sources for backup of various memories, power assist for an automobile, a train or the like, load leveling, rush current and UPS (Uninterruptible Power Supply) and so on have been developed and practically used. Performance requirement to an electrochemical capacitor has been increasingly stricter. In particular, it is desired to develop a highly durable electrochemical capacitor with a large capacitance per a unit electrode volume. Furthermore, in the light of safety, decreased gas generation is also desired.
Here, electrochemical capacitors can be generally categorized into electric double layer capacitors and pseudo-double-layer capacitors, depending on a mechanism of charge accumulation. An electric double layer capacitor stores charge by utilizing positive/negative charge from electric double layers aligned in a solid-liquid interface by applying an electric field. On the other hand, a pseudo-double-layer capacitor stores charge by a redox reaction of an active material on an electrode. An electrochemically active metal oxide or conductive polymer is used as a material for an electrode in a pseudo-double-layer capacitor (redox capacitor, pseudocapacitor) utilizing nonpolarizable pseudocapacitance (redox).
Electrolytic solution used in an electrochemical capacitor can be categorized into aqueous electrolytic solution as aqueous acid solutions and nonaqueous electrolytic solutions composed of an organic solvent as a main solvent. Generally, an aqueous electrolytic solution is characteristic in that it has a lower internal resistance due to its higher electric conductivity than a nonaqueous electrolytic solution and is inexpensive. On the other hand, a nonaqueous electrolytic solution is characterized in that an operating temperature range is wider and corrosivity is lower in comparison with an aqueous electrolytic solution. Furthermore, a nonaqueous electrolytic solution has a higher decomposition voltage than an aqueous electrolytic solution. An energy density of an electric double layer capacitor increases in proportion to the square of a cell voltage (maximum charge voltage). Therefore, a nonaqueous electrolytic solution can be used as an electrolytic solution, to give an electric double layer capacitor with a high energy density. Based on this standpoint, an electric double layer capacitor using a nonaqueous electrolytic solution has been commonly used. In particular, an electric double layer capacitor prepared using a polarizable electrode containing an activated carbon has a capacitance per a unit electrode volume and excellent durability, and is advantageous in terms of cost.
A maximum charge voltage of an electric double layer capacitor using a nonaqueous electrolytic solution is generally 2.5 to 2.7 V. If a larger voltage than the limits is applied, the capacitor is unsuitable for practical use due to reduction in a capacitance and increase in an internal resistance.
Reduction in a capacitance and increase in an internal resistance are caused by deterioration of constituent members (polarizable electrodes, an electrolytic solution, an electrolyte, a current collector and so on) in an electric double layer capacitor due to an electrochemical reaction. Specifically, it is known that in the positive electrode side, surface functional groups present in an activated carbon such as carboxyl and hydroxyl groups are decomposed to generate gases such as carbon dioxide and carbon monoxide. It is also known that a trace amount of water (residual water) contained in an electrolytic solution or activated carbon is decomposed to generate a strong acid, which causes corrosion of a current collector. In the negative electrode side, residual water is reductively decomposed to generate hydroxide ions, which promote decomposition of an electrolytic solution or electrolyte, inducing formation of insoluble polymer substance and generation of gases such as propylene, carbon dioxide, carbon monoxide and ethylene. Then, it is known that these substances occlude pores in a polarizable electrode, causing capacity reduction (Non-patent reference No. 1).
As well known, deterioration of an electric double layer capacitor can be inhibited by heating an activated carbon under an inert gas atmosphere to reduce surface functional groups in an activated carbon such as carboxyl and hydroxyl groups which are to initiate a deterioration reaction. However, even when an activated carbon with a reduced number of surface functional groups is used as a polarizable electrode, deterioration of an electric double layer capacitor cannot be adequately prevented. Furthermore, excessive removal of surface functional groups leads to reduced miscibility of activated carbon powders used for a polarizable electrode with a binder, so that formation of a polarizable electrode becomes difficult, resulting in reduction in productivity. Thus, various methods have been investigated for improving durability of an electric double layer capacitor on the basis of the above-mentioned deterioration mechanism.
Patent Reference No. 1 has described an electric double layer capacitor in which a capacitor element having a polarizable positive electrode, a polarizable negative electrode, and a separator disposed therebetween and containing a nonaqueous electrolytic solution is placed in an exterior case, wherein a solid base is contained in the exterior case. The method described in Patent Reference No. 1 is a method for improving durability by capturing impurity ions which accelerate decomposition of, for example, an electrolytic solution. Specifically, the method is a method for capturing protons generated by decomposition of residual water by a solid base such as amorphous silica-alumina and magnesia placed in a cell. It is described therein that the method prevents acidification of an electrolytic solution, improving durability of an electric double layer capacitor.
Patent Reference No. 2 has described an electrolytic solution additive for an electric double layer capacitor, which is made of an electrolytically polymerizable polymer precursor. According to the method described in Patent Reference No. 2, first, pyrrole, aniline and indole as electrolytically polymerizable polymer precursors are added to a nonaqueous electrolytic solution. Then, initial charge to the electric double layer capacitor initiates electrolytic polymerization of the polymer precursor contained in the electrolytic solution to precipitate a polymer on electrochemically active points present in the electrode in the positive electrode side, so that the active points are covered by the polymer. Thus, electrochemically active points which are believed to cause deterioration of the electrode are inactivated, so that deterioration of the electrode is prevented.
Patent Reference No. 3 has described an electric double layer capacitor comprising an electrolytic solution containing an electrolyte, an organic solvent and a triazole derivative, whereby corrosion of a collective electrode of electrodes, deterioration of an electrode binder and decomposition of an electrolytic solution can be prevented to give an electric double layer capacitor which has excellent high-temperature load properties and thus manifests higher voltage resistance. Patent Reference No. 3 has listed benzotriazole derivatives and triazole derivatives having an amino group as a triazole derivative used for an electrolytic solution.
Patent Reference No. 4 has described an electric double layer capacitor comprising an electrolytic solution containing γ-butyrolactone and one or two or more additives selected from anthraquinone, triazole and benzotriazole. It is therein described that the electric double layer capacitor prevents a reaction of water in the electrolytic solution with a current collector to prevent deterioration of an electrolytic solution and thus can be used at a voltage of 3 V or higher.
However, in an electric double layer capacitor prepared as described in Patent Reference Nos. 1 to 4, there is room for improvement in a capacitance maintenance rate and gas generation and durability is not completely satisfactory.